This invention relates to devices having surface features on Group III-V compound semiconductors and, more particularly, to preserving the integrity of such features when they are heated.
Surface features on semiconductors (and other materials) are used to affect the electrical, acoustical or optical properties of devices incorporating the semiconductor. Optical gratings, for example, are an array of parallel grooves formed on the surface of a semiconductor to reflect or filter light of predetermined wavelengths. In one specific application, a distributed feedback (DFB) Group III-V compound semiconductor laser, an optical grating serves as a filter which allows the laser to operate in only a single longitudinal mode (i.e., at a single frequency). To this end, the amplitude and periodicity of the grating are critical. The grating in such lasers is typically formed on the surface of a substrate or epitaxial layer and is then subjected to relatively high temperatures during subsequent growth of epitaxial layers over the grating. The effect of high temperatures during epitaxial growth is to deteriorate the grating shape (i.e., amplitude, spacing) which of course degrades the single-frequency performance of the laser.
Although grating degradation has been couched in terms of subjecting the grating to the temperatures of epitaxial growth, even if such growth were not employed in a particular device application (e.g., in the case of a grating on the top surface of an optical waveguide layer), other high temperature processing steps might also adversely affect the grating's characteristics.